<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>div固定定位</title>
    <style>
        #body_id{
            background: blue;
        }
        #div_id{
            width: 100px;
            height: 100px;
            background-color:red;
        }
    </style>
</head>
<body id="body_id">
    <div id="div_id">
        背景图固定定位:背景属于元素的一部分
        div固定定位:div独立于其他元素存在
    </div>
</body>
</html>